Considering the mouse arduino firmware was posted on August 29th, and this post was made on August 23rd, unless they had a time machine that would have been impossible.
Besides, attempting to solve problems in different ways is how we learn things and sometimes how we come up with even better solutions. A purely software based solution could be interesting as well, as it could probably be made to work on other Raspberry Pi devices, and perhaps other Linux systems in general. The arduino firmware method is a Gameshell specific option.